Photothermographic imaging systems are those imaging materials which, upon first being exposed to light in an imagewise fashion, produce an image when subsequently heated. The exposure to light or other radiation photoactivates or photodeactivates a component in the imageable element and subsequent heating causes an image forming reaction to differentially occur in exposed and unexposed regions.
Photothermographic imaging systems of the dry silver type are described in U.S. Pat. Nos. 3,457,075; 3,839,049 and 3,994,732. These imageable systems comprise a silver source material (usually an organic silver salt, e.g., a silver salt of an organic long chain fatty carboxylic acid, or a complexed silver salt), silver halide in catalytic proximity to the silver source material, a reducing agent for silver ion, and a binder. It is because the exposure and development of the imaging systems occur without using water, that these materials are often referred to as dry silver, light-sensitive materials.
In order to improve the sharpness or definition of photographic images an antihalation layer is often incorporated into photosensitive compositions. To be effective, the active ingredient in the antihalation layer will absorb at the wavelengths at which the photosensitive composition is sensitive. The longer the path length of the light in the layer of light-sensitive composition, the greater the attenuation. Therefore, scattered light is attenuated or absorbed to a larger extent than light which impinges directly on a light-sensitive crystal. As a result, although the overall speed of the composition is reduced slightly, scattered light and other light rays which are liable to produce a blurred image are preferentially absorbed and so the overall definition and sharpness of images produced in the layer are increased.
Antihalation compounds, known in the art as acutance agents, are dyes that are frequently incorporated into photosensitive systems. Preferably they are heat labile in the system, that is to say, they are degraded by the heat development of the photothermographic composition to one or more compounds which are substantially colorless. The exact mechanism of this reaction is not known. Such acutance agents are disclosed in, for examples, in U.S. Pat. No. 4,308,379.
British Patent Specification No. 1,261,102 discloses a transparent heat-developable photosensitive sheet material in which acutance is improved by incorporating relatively large proportions of colored material in a layer separate from the sensitive coating, which layer may be removed in a dry stripping process. On page 2, lines 28 to 44, methods are taught for stripping the color layer from the construction, such methods involving use of a pressure-sensitive adhesive tape on a corner or edge, or more effectively, supplying a thin coating of thermoplastic adhesive over the color layer and pressing the coating into contact with a sheet of paper during the required heat-development of the latent image. It is evident that the strippable layer was removed with difficulty.
A resistively heatable photothermographic element is disclosed in pending U.S. patent application Ser. No. 352,648, filed Feb. 26, 1982. The photothermographic element is provided with a two-layered strippable coating which has electrical resistivity in the range of 60 to 1500 ohms/square. The elements may be exposed to radiation and then thermally developed by applying a voltage across the strippable coating which becomes resistively heated. After development, the strippable coating may be removed.
Dry-strippable layers (which are adhered to glass or metal, for example) are known in the art. U.S. Pat. No. 3,619,335 relates to a unitary laminate comprising a backing layer which incorporates a radiation absorber, such as carbon black, dyes, and high atomic weight metals. The flexible polymeric film is strippably adhered to the backing layer by an intermediate adhesive layer. No mention is made in this patent of a laminate being useful in a photothermographic element.